When a transistor having an inductive load is turned off, that is, switched from a conducting state to a nonconducting state, the sudden reduction of current flowing through the inductive load causes its magnetic flux field to collapse. The collapsing flux produces a back electromotive force (EMF) of voltage across the inductor at such a polarity as to generate a current to oppose the changing flux. In high magnitude loads of the type commonly encountered in motor control, for example, and switching times on the order of one microsecond or less, the back EMF applied across the transistor may be on the order of magnitude of 100's of kilovolts at high peak instantaneous current levels. Because the presence of the inductive load tends to cause the locus of the operating point of the transistor during switching to define a path in the (I.sub.c, V.sub.ce) plane that is significantly displaced from the origin, the transistor is subjected to relatively high instantaneous power dissipation.
Several different methods have been provided for reducing surge currents and voltages in power transistors during inductive load switching. A common technique for snubbing the current surge imparted by the inductive load during transistor turn-on has been to connect an inductor in series with the collector of the transistor to reduce the time rate of change of current and a flyback diode across the snubber inductor to enable "freewheeling" of current generated by the snubber. Voltage surges impressed across the collector and emitter terminals of the transistor during turn-off switching are commonly limited by a Zener diode connected across the collector and emitter of the transistor or are suppressed by a snubber capacitor connected between the collector and emitter. Transient current and voltage control techniques of the limiter and snubber types are shown, for example, in U.S. Pat. Nos. 3,641,407 and 3,418,495 as well as in Kuecken, Solid State Motor Controls, Tab Books, June, 1978, pages 39-45.
Whereas current snubbing using a series inductor has been found to be satisfactory, voltage limiting using Zener diodes or other threshold device or suppressing using snubber capacitors have certain disadvantages. Zener diodes can reliably dissipate only a limited amount of power and are prone to failure. In order to maximize reliability in view of the large voltage surges created by the inductive kick during turn-off switching of heavy inductive loads, such as motors, very large, costly Zener diodes must be used in conservative design. Although snubber capacitors, which are more reliable and less costly than large Zener diodes, effectively suppress voltage transients by supplying a current path between the inductive load and ground during the turn-off switching transitions of the transistors, capacitor recharge currents flowing through the transistors during the turn-on transitions increase transistor power dissipation. In a voltage snubber circuit for a transistor operated inverter, for example, of a type wherein transistors are switched sequentially in positive and negative legs of the inverter to supply bidirectional current having a predetermined waveform to an inductive load, turn-off transient voltages impressed across the transistors are suppressed by current flow established between the load and ground through the snubber capacitor. During the transistor turn-on transitions, however, the snubber capacitor recharge current flowing through the inverter transistors in addition to load current significantly increases the instantaneous power dissipation in the transistors. The magnitude of the capacitor recharge current is substantial since the snubber capacitor must be large to maintain low the dV/dT transient voltages impressed across the switching transistors in a high current, short switching time environment.
One object of the invention, therefore, is to provide an improved snubber circuit for suppressing the surge voltage applied across a transistor under inductive loading during turn-off.
Another object is to provide a voltage snubber circuit wherein there is no flow of significant capacitor recharge current through snubber protected transistors during turn-on.
An additional object is to provide a gated snubber circuit connected to the output of a push-pull type inverter wherein the snubber is gated on in response to transistor turn-off transitions in the positive and negative legs of the inverter.